The present invention relates to a three-dimensionally structured fibrous web and a method for manufacturing a three-dimensionally structured fibrous web.
By xe2x80x9cthree-dimensionally structuredxe2x80x9d is meant here fibrous webs in which the orientation and the spatial coordination of the individual fibers with respect to each other in any given surface plane diverge from those in the next closest surface plane.
In particular, the present invention relates to the field of fibrous webs, in which nonwoven fabric layers are bonded on both sides to at least one layer made of a scrim, a lattice, or a netting and a method for its manufacture.
U.S. Pat. No. 4,302,495 showns fibrous webs.
One or a plurality of layers made of discontinuous, thermoplastic polymer fibers and one or a plurality of layers composed of an open-mesh netting made of coarse, thermoplastic, continuous melt-blown fibers, which cross each other at a preestablished angle, are bonded to each other by thermal fusing, either continuously or in spot fashion, to produce a web having a uniform thickness. The randomly running short fibers have a diameter of between 0.5 and 30 xcexcm at a weight per unit area of 10 to 15 g/m2. Both the combination, lattice/microfiber layer/lattice, as well as microfiber layer/lattice/microfiber layer are described. A material that may be preferred for both the microfibers as well as the filaments of the lattice is polypropylene. A web of this type may have a very high tensile strength, together with a porosity that can be precisely adjusted. The melt-blown microfiber layers determine the external appearance and, for example, the filtering properties, whereas the thermoplastic netting(s) aid in reinforcement, controlling the porosity, and, if appropriate, simulating the appearance of a woven textile fabric. Therefore, the material may be suitable not only for use as filters, but also as a sterile packing material in surgery. Further application areas may be chemically inert filter media or non-wettable, light-weight, thermal insulating layers for clothing, gloves, or boots.
The thermal bonding of the layers to each other may be carried out under pressure, for example, between heated rolls, one of which having the appropriate engraving if spot-bonding is desired. In addition, heat radiation may be applied before the heating is carried out between the rolls. The level of the heating effect may be set so that the fiber materials soften without undergoing a temperature increase to the level of their crystalline melting point.
It was discovered that fibrous webs of this type may not stand up to pressure spikes or other powerful mechanical forces over a longer period of time without significant compaction, if, when packed, stored for extended periods, and transported, they are exposed to high pressures and temperatures up to 60xc2x0 C., which is entirely usual in a shipment to tropical countries.
In addition, three-dimensional webs are disclosed in U.S. Pat. No. 4,522,863; British Patent 1 331 817; U.S. Pat. No. 5,525,397 and WO 98/52458, the webs being composed of a scrim, lattice, or netting and being bonded to nonwoven fabric layers on both sides.
An objective of an exemplary embodiment and/or exemplary method the present invention is to indicate a three-dimensionally structured fibrous web which stands up to pressure spikes up to 1 psi acting perpendicular to the surface plane without being destroyed, even at temperatures up to 60xc2x0 C.
According to an exemplary embodiment of the present invention, at least two nonwoven fabric layers are bonded, in each case, to one scrim layer. The nonwoven fabric layers are made up of fibers that are bonded to each other mechanically and/or thermally and that, in the surface direction, possess a fold-like pattern in the form of geometric, repeating elevations or undulations.
Present in the above exemplary embodiment of the present invention is at least one thermoplastic scrim, lattice, or netting layer having continuous filaments crossing each other and bonded at the crossing points by fusion, the filaments having a thickness of 150 to 2000 xcexcm between their crossing points, and having thickenings at the crossing points of up to seven times these values. For reasons of simplicity, this layer hereinafter is always termed a scrim, even if other structures having crossing individual filaments are at issue.
The mesh size of the scrim of the above exemplary embodiment, i.e., the distance in each case between two adjacent filament crossing points in the longitudinal direction, multiplied by the corresponding distance in the transverse direction, is 0.01 to 9 cm2, assuming that the filament crossing points in the longitudinal as well as in the transverse direction have a distance from each other that is not less than 0.10 cm.
The specific bond between fiber layers and the scrim layers may be of the spot type.
In further exemplary embodiments of the present invention, the continuous filaments of the scrim are made up, for example, of polyethylene, polypropylene, polyamide-6, polyamide-6.6, polybutylene terephthalate, polyethylene terephthalate, polyester elastomers, copolyesters, copolymers made of ethylene and vinyl acetate or of polyurethane.
In a further exemplary embodiment of the present invention, the scrim is made up of a netting that is biaxially elongated. The elongation in the direction of both filament patterns is carried out in accordance with known methods in the longitudinal direction by by passing through the gap between a slower moving and a more rapidly moving roll, the elongation ratio thus being determined by the ratio of the more rapidly moving to the more slowly moving rolls. In the transverse direction, the elongation is carried out using an expanding tenter frame.
This known method brings about a reduction in the thickness of the filaments between the mutual crossing points and therefore a reduction in the weight per unit area of up to 95%.
According to an aspect of a further exemplary embodiment of the present invention, it is possible to carry out the double-sided covering of the scrim using nonwoven fabric such that each nonwoven fabric layer has different properties with respect to the configuration of its folds or with respect to its inherent properties, such as weight per unit area, type of fiber, and fiber bonding.
In general, in selecting the parameters for the nonwoven fabrics with respect to composition, type of fiber, fiber bonding, and fiber orientation, the worker skilled in the art is guided by the properties known to him that these layers are supposed to have. In the interest of a high inherent rigidity of the elevations and undulations, it is necessary for the nonwoven fabric fibers to be intensively bonded to each other.
If the fibers are fixed using a bonding agent, a bonding agent having a hard grip is preferable, because in this way the inherent rigidity and mechanical resistance of the fibrous web is increased overall.
It is believed that it is advantageous if the distance from one filament crossing point to the next one in the scrim, as well as the degree of elongation and the filament strength in the longitudinal and transverse directions, are approximately the same, because in this way, after the shrinking process, elevations are produced having a circular base cross-section. These have proven to be the most resistant to pressure loads exerted perpendicular to the surface plane.
Depending on the starting material selected, multilayer fibrous webs may be produced having weights of 20 to 3000 g/m2. Products having lower weights per unit area are-suitable, for example, for layers in diapers that absorb and distribute liquid, such as have up to 3000 g/m2 for high-volume filter matting, which have a high retention capacity for the filtrate.